The meeting was a coming-out party of sorts for scientists interested in the biological implications of the chemical changes occurring as the oceans absorb huge and growing amounts of atmospheric carbon dioxide. Just 8 years ago, an inaugural symposium on the topic in Paris drew only 125 researchers from 20 nations; this year, more than 550 scientists from 40 nations showed up. The field is getting “much bigger and more competitive,” Gattuso says.

Acidification researchers are also shifting their focus. To date, many experiments have involved simply plopping sea creatures into laboratory tanks full of acidified water for a few days or months to see how they respond. Many species suffer, researchers reported. Fish and shellfish larvae exposed to more acidic waters, for example, often fail to thrive: They don’t grow as big or live as long as those born in more alkaline waters. But some species show substantial resilience, reported biologist Sam Dupont of the University of Gothenburg, Kristineberg, in Sweden. After he used acidic water to completely dissolve the shells of developing sea urchins, for instance, the urchins were able to regrow them and live normally once they were returned to normal seawater.

Such limited studies, however, “can’t really tell you whether a species has the capacity to adapt to acidification, or how pH changes affect a larger ecosystem,” says marine scientist Gretchen Hofmann of the University of California, Santa Barbara.

One approach to leaping those limitations is to go back to the future, by looking for times in the fossil record when ocean ecosystems experienced similarly dramatic carbon dioxide–driven changes. One popular candidate, known as the Paleocene-Eocene Thermal Maximum (PETM), occurred 55 million years ago during rapid global warming (Science, 18 June 2010, p. 1500). Increasingly corrosive bottom waters appear to have helped drive many bottom-dwelling species extinct during the PETM, reported paleontologist Paul Bown of University College London. But what happened at the ocean’s surface is less clear. The fossil record suggests that many species of phytoplankton—the tiny plants at the base of the marine food chain—also disappeared but were replaced by other species, with little change in overall diversity.

But such coarse measures can’t tell you how ancient acidification might have affected reproduction or growth patterns in these marine communities, Bown says. To get that more detailed view, Bown and his colleagues have been analyzing some exquisitely preserved fossils of PETM phytoplankton called coccolithophores, which surround themselves with shieldlike plates of shell. By studying some closely related living species, the researchers found that they could estimate ancient coccolith growth and reproduction patterns by painstakingly counting the plates on individual fossils. (The number increases as the organisms grow.) So far, preliminary studies haven’t found deformed shells or other dramatic signs of lower pH, but Bown cautions against taking that as a sign that modern acidification won’t be a problem. Change in the PETM moved “much, much slower than today,” he says.